A laser diode is generally used as a writing light source for a printing device such as a printer or copier. The laser diode used as an optical writing light source requires that a light emission amount be kept to a constant value for making the density of a generated image constant. Thus, JP2001-138566A, for example, discloses, for keeping a light amount of a laser diode (LD) constant, detecting at a photodetecting device (PD) housed in the same package as the LD light from the LD, and using a monitoring current generated at the detecting PD to control the light amount to be constant. In other words, the monitoring current generated at the PD due to the detected light passes through a light-amount setting variable resistor so as to be converted to a monitoring voltage, which is input to a comparator so as to be compared with a reference voltage and controls the voltage of a hold capacitor. Specifically, with the comparator output connected to a control circuit, the process is performed such that, with a sample-and-hold signal input to a control circuit in the sample mode, the monitoring and reference voltages are compared so that when “the monitoring voltage>the reference voltage”, a current passes through the hold capacitor from a constant discharging current source so as to cause a voltage drop across the hold capacitor; when “the monitoring voltage<the reference voltage”, a current passes through the hold capacitor from a constant charging current source so as to cause a voltage rise across the hold capacitor. With the sample-and-hold signal input to the control circuit in the hold mode, the constant current source is disconnected from the hold capacitor. In this way, the terminal voltage of the hold capacitor is input to a differential amplifier, causing a current supplied to the LD to be increased or decreased according to the difference with the reference voltage. Such a series of control loops as described above allows the light emission amount of the LD to be kept constant.
Recently, a light-emitting device has also been developed which has a large number of light-emitting points in one package, and is used as a writing light source for a printing device. For example, JP2003-266774A discloses an image-forming apparatus using a VCSEL with 32 light-emitting points.
However, with the control method as disclosed in JP2001-138566, where a hold capacitor is charged or discharged during a sample period, and placed in a hold mode at other times so as to keep the light amount constant, there may be a problem of a light amount decrease caused by a decrease in a terminal voltage due to a leakage current of the hold capacitor. Moreover, the sample period needs to be placed at a time other than an image writing period. With an arrangement using a light source with a large number of light-emitting points, as disclosed in JP2003-266774A, a light-amount control of all the light-emitting points cannot be performed within one scan period. Therefore, there is a further problem of a light amount decrease caused by a decrease in the terminal voltage of the hold capacitor.
Now, a problem to be solved by the present invention is to ensure control sufficient to keep the light amount constant, and to ensure control sufficient to keep the light amount constant even with, especially, a light source with a large number of light-emitting points.